This invention relates to apparatus for pulverizing, i.e., grinding, material, and more specifically to a vane wheel arrangement that is particularly suited for embodiment in a bowl mill wherein the vane wheel arrangement is operative to effect a primary classification of the pulverized material.
It has long been known in the prior art to provide apparatus that is suitable for employment for purposes of effecting the grinding, i.e., pulverization, of materials. More specifically, the prior art is replete with examples of various types of apparatus that had been used heretofore to effect the grinding of a multiplicity of different kinds of materials. In this regard, in many instances discernible differences of a structural nature can be found to exist between individual ones of the aforesaid apparatus. The existence of such differences is in turn attributable for the most part of the diverse functional requirements that are associated with the individual applications in which such apparatus are designed to be employed. For instance, in the selection of the particular type of apparatus that is to be utilized for a specific application one of the principal factors to which consideration must be given is that of the nature of the material that is to be ground in the apparatus.
Coal is one such material wherein there exists a need to grind the material in order to render it suitable for use in certain applications. Furthermore, fossil fuel fired power generation systems represent one such application in which it is desired to employ coal, as the source of fuel therefor, and wherein a requirement exists to grind, i.e., pulverize, the coal in order to render it suitable for use for this purpose. To this end, coal has long been recognized as being one of this nation's most abundant sources of fuel. At one time earlier in this century, much of the nation's energy needs were being met thorugh the use of coal. Then, in the degree to which coal was being employed to generate power a decline set in. Much of this decline stemmed from the increased usage of oil and gas as sources of fuel. More recently, the power being generated from the burning of oil and gas has been supplemented by the use of nuclear fuel for power producing purposes. However, with the advent of the oil embargo in the last decade, which was accompanied by a sharp increase in the price of oil and the existence of restricted oil supplies, and the increased concern, which has since been expressed over the rate at which the world' s known oil reserves are being depleted, coal has begun to regain some of the favor, which it once had as a source of fuel to meet the nation's energy needs. To some extent, this has been evidenced in a number of orders, which have been placed in recenlty passed years, for power generation systems that are to be coal-fired as well as the extent to which increased interest has been shown in effecting the conversion of existing oil and gas fired power generation systems to coal fired systems.
For purposes of the discussion that follows, the coal fired systems referred to above are considered to consist of essentially the following major operating components: a coal feeder, apparatus for pulverizing the coal, a distribution system for distributing the coal after the pulverization thereof, a furnace in which the coal is to be burned, and the requisite controls for effecting the proper operation of the coal fired power generation system. Of particular interest herein is that portion of the coal fired system, which has been identified above as the apparatus for pulverizing the coal. Coal pulverizing apparatus are not new. They have been know to exist in the prior art for more than half a century. Furthermore, many improvements in the construction and/or mode of operation of coal pulverizing apparatus have been made during this period.
There are a number of features that it is advantageous for any coal pulverizing apparatus to possess, but particularly those which are designed for employment in a coal fired power generation system. Reference is had here to features such as reliability, low power consumption, minimum maintenance and wide range of capacity. In addition, such apparatus advantageously should also be characterized by quiet operation, integrated lubrication systems, convenient adjustment and control of coal flow and fineness, and the ability to handle the high temperature air that is required for high moisture coal.
One particular type of coal pulverizing apparatus, which is to be found in the prior art, that is advantageously characterized by the embodiment therein of the above recited features is an apparatus, most commonly referred to in the industry by the name bowl mill. The latter apparatus obtains its name by virtue of the fact that the pulverization, i.e., grinding, of the coal which takes place therein is effected on a grinding surface that in configuration bears a resemblence to a bowl. Reference may be had by way of exemplification to U.S. Pat. No. 3,465,971, which issued Sept. 9, 1969 to J. F. Dalenberg et al., and/or U.S. Pat. No. 4,002,299, which issued Jan. 11, 1977 to C. J. Skalka, both of the latter patents being assigned to the same assignee as the instant application, for a teaching of the nature of the construction and the mode of operation of a prior art form of bowl mill that is suitable for use in a coal fired power generation system to effectuate the pulverization of the coal that is to be burned as fuel therein. As taught by the aforereferenced patents, a bowl mill essentially consists of a body portion in which a grinding table is mounted for rotation, a plurality of grinding rollers that coact with the grinding table to effect the grinding of coal interposed therebetween, coal supply means for feeding to the interior of the bowl mill the coal that is to be pulverized, and air supply means for supplying to the interior of the bowl mill the air required in the operation of the latter. In accordance with the mode of operation of such a bowl mill, the coal, which enters the bowl mill, is pulverized by virtue of the coaction of the grinding rollers with the grinding table. After being pulverized, the coal particles are thrown outwardly by centrifugal force whereby the particles are fed into a stream of air that is entering the bowl mill. The stream of air, which now contains pulverized coal particles, flows through a tortuous path that is established in part by the positioning within the bowl mill of a suitably supported deflector means. As the stream of air and coal particles flows along the aforementioned tortuous path, the sharp turns contained therein effects the separation of the coarse coal particles from the air stream. These coarse coal particles are then suitably returned to the grinding table for further pulverization, while the fine coal particles are carried through the bowl mill in the air stream, and exit therefrom along with the air.
In a conventional coal fired power generation system, a multiplicity of bowl mills of the type shown in the aforereferenced patents would commonly be employed for purposes of satisfying the requirements of the system for pulverized coal. By way of example, the capacity of each of the individual bowl mills might be on the order of 100 tons per hour of coal.
Although bowl mills constructed in accordance with the teachings of the aforereferenced patents have under actual operating conditions provided adequate performance to date, a need has nevertheless been evidenced for improvements to be made therein. More specifically, prolonged operation of this type of bowl mill has revealed the existence of several conditions of an undesirable nature that can arise during the use thereof. One of these is related to the need for and the manner in which a primary classification is had within the bowl mill of the material that is being pulverized therewithin. As employed herein the term primary classification is intended to refer to the separation of pulverized material from the air in which such material is entrained. In particular, reference is had here to that separation of pulverized material which occurs as a consequence of causing the air within which the pulverized material is entrained to follow a tortuous path through the bowl mill whereby in the course of changing directions of flow the larger of the particles of the pulverized material lose their momentum and are made to return to the surface of the grinding table whereat they are subjected to further pulverization.
In accordance with the teachings of the prior art, it has been known to employ a separator body liner design in a bowl mill for purposes of accomplishing the aforedescribed primary classification of pulverized material therewithin. By way of exemplification, such a bowl mill comprises the subject matter of U.S. Pat. No. 4,234,132 which issued on Nov. 18, 1980 to one of the two co-inventors of the present application and which is assigned to the same assignee as the present application. Although a bowl mill equipped with such a separator body liner design has proven to be adequate in terms of accomplishing the subject primary classification of the pulverized materials in a bowl mill disadvantages nevertheless are associated with the employment thereof. Namely, the air deflector means of such a bowl mill is so located therewithin for purposes of accomplishing the aforesaid primary classification of pulverized materials as to be positioned in overhanging relation to the grinding table of the bowl mill. As a consequence, access to the internal workings of a bowl mill equipped with such air deflector means is limited thereby by the positioning thereof. In addition, damage can be had thereto when incidents involving tramp iron occur. Finally, it would be desirable to achieve a better wear distribution pattern insofar as concerns those members mounted within the bowl mill which are designed to be used to direct the flow of air through the bowl mill. A need has thus been evidenced for a new and improved means suitable for employment in a bowl mill which would be operative when so positioned therewithin to effect the primary classification of material being pulverized within the bowl mill.
It is, therefore, an object of the present invention to provide a new and improved primary classifier means that is suitably constructed so as to be employable in a bowl mill.
It is another object of the present invention to provide such a primary classifier means for bowl mills which does not inhibit access from being had to the internal workings of the bowl mill.
It is still another object of the present invention to provide such a primary classifier means for bowl mills which includes vane means suitably supported on the circumference of the rotatable grinding surface of the bowl mill and operative to establish a direction of flow to the air that passes through the interior of the bowl mill.
A further object of the present invention is to provide such a primary classifier means for bowl mills which also includes a converging/diverging orifice means mounted in spaced relation to the rotatable grinding surface of the bowl mill and operative to further establish a direction of flow to the air that passes through the interior of the bowl mill.
A still further object of the present invention is to provide such a primary classifier means for bowl mills having vane means and converging/diverging orifice means that function in conjunction with one another to cause the larger of the particles of pulverized material which are entrained in the stream of air that flows through the bowl mill to be separated from the air stream and to be returned to the rotatable grinding surface for additional pulverization.
Yet another object of the present invention is to provide such a primary classifier means for bowl mills wherein the wear surfaces of the vane means and the converging/diverging orifice means are formed of a highly abrasive resistant material.
Yet still another object of the present invention is to provide such a primary classifier means for bowl mills which is suitable for employment in newly constructed bowl mills as well as being equally suitable for employment in retrofit applications.